1. Field of the Invention
The present invention relates to wavelength division multiplexers (WDMs) and, in particular, to multi-window WDMs (MWDMs) with fused-fiber Mach-Zehnder Interferometers (MZIs) and methods of making the same.
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. The use of wavelength division multiplexers (WDMs) provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. In addition, WDMs can be used in fiber optic communication systems for other purposes, such as dispersion compensation, noise reduction, and gain flatting, i.e., maintaining a uniform gain within the usable bandwidth for erbium doped amplifiers.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, so that light of two wavelengths (i.e., 1310 nm and 1550 nm) entering the input terminals of the first and second fibers, respectively, are multiplexed onto a single fiber. The coupling ratios for the two channels (the signals at 1310 nm and 1550 nm) exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks (or windows) within the passband. Information carried by the two signals along the single fiber is then demultiplexed at the WDM outputs.
Multi-window WDMs (MWDMs) have two or more peaks of amplitude as a function of frequency (or operational windows) for each channel within a passband. MWDMs can also be made using BTF technology by twisting two optical fibers together, fusing the center portion together, and pulling the fibers until a desired multi-window transmission spectrum appears at a monitored fiber output terminal. Such a long-tapered-fusing technology is discussed in commonly-owned U.S. patent application Ser. No. 08/799,582, entitled "Apparatus and Method of Making a Fused Dense Wavelength-Division Multiplexer", now U.S. Pat. No. 5,809,190 which is incorporated by reference herein in its entirety. However, it is difficult to make a high performance fused-fiber MWDM with a narrow band and narrow window spacing utilizing BTF technology because the MWDM may have a very long fused coupling region, which makes the MWDM extremely fragile. Also, the MWDM may exhibit long-term instability against environmental changes and poor uniformity over time.
Accordingly, a structure and method are desired which overcomes the disadvantages discussed above with respect to fused-fiber MWDMs.